Moving object pulse echo selection circuit for radar systems



July 21, 1953 A. e. EMSLIE 5 9 MOVING OBJECT PULSE ECHO SELECTIONCIRCUIT FOR RADAR SYSTEMS Filed April so, 1946 2 Sheegg-Sheet 1 lo DELAYLINE AMF! 8 (20 l2 l6 COMPARISON '2 RECEIVER CIRCUIT DELAY LINE AMP.

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AT TOHWE Y July 21, 1953 A. e. EMSLIE MOVING OBJECT PULSE ECHO SELECTIONCIRCUIT FOR .RADAR SYSTEMS Filed April 30 1946 3 wl "B w D R R n TE TuMI E; \H X P3 J "mm T -2 Q Q T A m R D F E T V I m 4 R B 2 P 0 w ML -QLfi L I 1 L 11 E T n m m D R A0 B0 C0 D0 0 F 0 GO HG INVENTUR ALFRED GEMSLIE flaw-4M. Q/Aep,

A TTOIiWE Y Patented July 21 1 953 l pairs f attest-i MovrNG censor PULSscno SELECTION CIRCUIT FGR RADAR SYSTEMS Alfred G. Emslie, Boston, 7Mass, assignor, by mesne assignments, to the United States of America asrepresented by the Secretary of War Appiication April 30, 1946, SerialNo. 665,993

13 Claims. (c1. sis-rm This invention relates to electrical apparatusand more particularly to comparison circuits employed in moving targetradio object locating systems.

- Radio object locating systems for locating the 1 position of objectsin spacehave been employed for a number of years. In this type of radiosysterm a visual indication is usually provided that displays in map orgraphic form azimuth, eleva: tion or range data of all objects bothfixed and moving within the region of space scanned by the system.

More recently radio object locating systems have been developed thatpresent only data relating to moving objectspthat is, data appearing onthe visual indicator represents only objects moving with respect to theradio system or moving with respect to some reference such as the earth.Since all objects both fixed and moving reflect energy'that will bedetected by the system re ceiver, it is necessary to separate radioechoes representing moving objects from radio-echoes representingstationary objects. Thisis done in the receiver channel of the radiosystem The separation of the two types of radio echoes is usuallyaccomplished in the following manner. The returning radio echoes arecombined with a reference oscillation that is fixed in phase withrespect to the pulses of radio energy transmitted by the system,Individual echoes in a train of echoes returned from any stationaryobject will always bear a fixed phase with respect to this referenceoscillation since the distance between the radio system and the echoreturning object is constant. The combination of the train of echoesfrom any specific stationary object with the reference oscillationresults in a train of signals of fixed amplitude.

The distance from the radio system to moving objects is not constant;therefore, echoes returned from'moving objects will have a variablephase withrespect to the reference oscillation. It can be shown that thesignals resulting from the combination of the reference oscillation witha train of echoes from a moving object will result in a train of signalshaving a cyclic variation in amplitude. echoes from fixed and movingobjects have thus been established; signals representing fixed objectshave a constant amplitude while signals representing moving objects arevariable in amplitude. 1 I

To separate the two types of signals a circuit known in the art as acomparator is normally employed. In one of its simple forms it comprisesa linear mixer of the mixerand signals as they are in this example.

havingtwo signal inputs. Undelayed video signals .are applied toone'inputthat: have been inverted and delayed by a period equal to, thetime interval between successive transmitted pulses are ap plied to asecond input. It'can be seen that, since successive echo pulses fromJfixed objects are equal in amplitude, no output signal will be obtainedfrom the mixer if successive signals are applied simultaneously to theinput of the mixer On the other hand, a signal will be obtained from the.outputof the mixer when successive signals representing a moving objectare simultaneously applied to the input of the mixer due tothedifierence of ampli-- tude of these successive signals. The signalsin the output of the mixer are substantially similar in character tosignals normally output of a radio object locating system receiver;-therefore, vthey'may bexapplied directly to the system indicator ifdesired and onlyrindications representing moving objects will thenappear on the screen thereof.

. a supersonic delay? line. *mitter must also be controlled orstabilized; by

I fore, to provide a Distinguishable characteristics for One of the mostimportant, factors in the compariscn phase is that the delay of signalsapplied to the mixer must beexactly equal to the time between successivetransmitted exploratory pulses. I This delayis usuallyaccomplished withThe radio pulse trans-- the same or a. similar delay line. Since thetransmitter pulsing'is controlled by a seriesofaccurately spaced triggerpulses, the so-called hard tube type of modulator" must be employed. Itis usually more convenient to'use a .so-ca1led spark gap modulatorbecause of its simplicity.v It has been impossible to use this type ofmodulator heretofore due to the variation in time spacing betweensuccessive transmitted pulses when this circuit is used.

It is an object of the present invention, therecircuit that will permitthe use in a'rnoving object radio detection systemiof a pulsetransmitter the pulse repetition interval of which is not stabilize Itis a more particular object of this invention to. provide acircuit-thatwill permit the use of spark gap modulator'in-a moving object radiodetection system. I

A further object of the present invention is to provide a circuitthatwill compensate for the variation in the period between successivepulses transmitted by a moving objectradio detection system. 2 I. P

For a better understanding of the invention present in the 1 togetherwith other and further objects thereof,

reference is had to the following description to r be' read inconnection with the accompanying drawings in which:

Fig. 1 is a schematic drawing in block form of the present invention;

Fig. 2 is a more detailed drawing of a portion of the circuit of Fig. 1;and.

Fig. 3 is a series of typical waveforms that might be obtained atselected points in the circuit of Fig. 1. f

In Fig. 1 a receiver I supplies identicalsignals I to two delay lines l2and I4. Receiver [0 is the radio receiver that forms a part of the radioobject locating system. Delay line l2..is preferably of the supersonicdelay type and should provide a time delay that is longer than thelongest interval between any two successive exploratory pulsestransmitted by the radio system associated with this circuit. Delay'line14 provides a variable time delay, this time delay being'adjustable over7 a comparatively wide range. .Onepreferred-form of delay line l4 istheso-called positive ion delay line more fully described hereinafter, Theout put signals from delay lines 12 and 14 are applied throughamplifiers I6 and I8 to two inputs of a comparison circuit 23.Comparison circuit 28 combines the outputs from delay lines l2 and E4 ina manner to provide at the output22 a signal equal to the difference inamplitude of the two applied signals.

A modulator 24 provides activating signals for the radio systemtransmitter (not shown). Modulator 24 may be of any convenient type and,for purposes of illustration only, it may be assumed to be of the sparkgap type. -Modulator 24 supplies a trigger pulse to a delay line .26 andto a control voltage circuit 28 at the same time that the activatingsignal is applied to the transmitter. The time delay of line26 should beless than the minimum time spacing between suctential produced bycharging circuit 38. Many circuits of this type are well known in theart; therefore clamp circuit will not be described in detail. A positiveion delay line l4 comprises a cathode 42, an intensity grid 44, anaccelerating anode'46 which may also serve as a beam forming anode, anda collector anode 48.- Cathode 42 of delayline I4 is connected to theoutput' of voltage clamp 40 so that cathode 42 is maintained outputconnection 60, through a capacitor 62.

Output connection -63 is. connected to amplifier I8 of Fig. 1. I

In the circuit of Fig. .2, cathode 4.2 is held at a positive potentialwith respect to ground :by an amount proportional to the timeintervalbetween triggers applied at connections .30 and 32 respectively. .Themore positive the potential on path: 7

ode 42 the shorter will be the transit time of positive ions travelingfrom cathode "4.2 "tocol'lector anode 48. Video signals 'fromreceiyer.18 of Fig. 1 are applied to input connection :52 of Fig.

7 2 thus changing the potential of grid 44 cessive exploratory pulses.The output of delay line 26 is applied to a second inputconnection ofcontrol voltage circuit 28. The output signal from controlvoltagecircuit 28 controls the time delay of signals passing through thedelayv line I4.

- Fig. 2 illustrates in more detail the circuitry of control voltagecircuit 28 and delay line M.

In Fig.2 two inputs3il and 32 to a gate generator 34 provide means forintroducing signals from the output of delay line 26' and modulator .24respectively, of Fig. 1. The initial time of the voltage gates producedby generator 34 is de-. termined by signals applied'by way of connection30 and these same voltage gates are terminated by signals applied by wayof connection 32.. The length of the voltage gate produced by generator34 will, therefore, depend on the time interval between pulses in theoutput of delay line 26 and the next succeeding pulses in the output ofmodulator 24. The output of gate generator 34 controls a chargingcircuit 38. Charging circuit 38 may be any form of circuit that willproduce a voltage that is some selected function, not necesor waveformsshown in Fig. 3.

respect to cathode42 so that a group of positive ions is allowed to passfrom cathode 42 to 1001- lector anode 48 at each time that .a video:signei is applied to connection 52.. f v

The operation of the circuit shown in Fig. 1 may be best understcocl byreference to curves In Fig. 3 voltage waveforms from various points inthe circuit "of Fig. l have. been :plotted against a common time scalethat forms the abscissa .of .all curves. For convenience in the drawing,the time bases .of

i Figs. BGandBH-have been shifted to theleftassarily a linear function,of the time that this circuit is made operative by gategenerator 34. Forpurposes of illustration it will .be assumed that the output potentialof charging circuit 38 is a linear function ofthe length in time of thegate applied from generator.3 4. Avoltage clamp circuit 40 connected, tothe output of circuit 38 provides means for maintaining for a specifiedinterval a .potential egual to the maximum poindicated by diagonaldottedlines drawn between corresponding time points in Fig. 3Fia-ndFig.33G. The time scales of Figs. 3G and 3H are exactly the .same as thetime scales of the remaining waveforms in Fig.3. J

In Fig. 3A there is shown a series of trigger pulses namely T1,'T2, andT3 that'represen-t output triggers from modulator .24.

pulses shown in .Fig. 3A are applied through de The trigger lay line 26'to .controlvoltage circuit .28 and directly from modulator .24 to asecond input of control. voltage-circuit .28. Fig. .33 illustratestrigger pulses in the output of delay line 26. Pulses T1, T2, and T3correspond-"to pulses'Ti, T2, and T3 in Fig. 13A. The delay time betweenpulse T and .the corresponding pulse T13 herein shown as delay time D1,is equal to the time idelay in delay line 26. This time .delay should:be less than the minimum-spacing between any two successive pulses inFig. 3A. In this example pulse T1 inFig. 3B-activates control .voltagecircuit 2B andcauses gate generator 34 of Fig. 2 to be made operative.At-a slightly later time pulse T2 in Fig. 3A will cause generator 34 tobe made inoperative and thus limit thelength-of the gate produced bygenerator 34. The. length of the gate produced in this case isdesignated by the time C1 in Fig. 3. Pulse T2 will again activategenerator 34 and after a time interval designated herein as C2, pulse T3of Fig. 3A will deactivate generator 34. Fig. 30 illustrates the pulsesor to interval C1 while the second gate; 68 will-have,

in this case, a shorter time duration, this time duration beingequali-tothe time, interval Caiiltshould be understood that the lengthof .the'volt age gates produced by a generator 34 depend-only on therelative spacing of pulses in the output of modulator 24 and, therefore,the.lengthix.of

'liese gates will usually varyina random man-- Fig; 3D illustratesthe-output signalfrom chargingcircuittli of Fig. 2. As before stated.the maximum potential of the output signalifrom charging circuit 38 willdepend upon the length ner.

of the gate produced by generator-34. In this example signal of Fig. 3D.will have aihigherI amplitude V1 than the amp1it'ude'V2 of 'signal12' ofthis same figure due to the fact that gate T68 in Fig. 3C is of longertime duration than gate 68. Fig. 3E illustrates the signal output ofvolt-i age clamp circuit 40 and therefore thesignal applied to thecathode 42 of delay line I4 at the end of signal It, that is, after thetime of occurrence of pulse T2. The cathode 42 of=delay line I4 will beclamped at a potential V1 above ground.

V1 is equal to the maximum amplitude of signal It in Fig. 3D. 1 1 At atime corresponding to the time of-occurrence of pulse T3 in'Fig. "3'A,the potential on.

cathode 42 will drop to a potential V2 .above ground correspondingto'the maximum potential V: of signal 72 in Fig. 3D. 'Pausing briefly tonote the effect of this change in potential on delay line 54 it can beseen that the delay of signals passing through delay line It between thetime of occurrence of pulses'T2 and T3 will be less than the time delayof signals occurring between pulse T3 and the next succeeding pulse (notshown) due to the fact that cathode 42 is at a higher potential duringthe former time interval and therefore the velocity of the positive ionstraveling from cathode 42 to anode 48 will be greater.

Video signals in the output of receiver ID of Fig. 1 are illustratedinFig. 3F. Pulses P1, P2, and P3 represent successive transmittedexploratory pulses while pulses E1; E2, and E3 represent a series ofsignals returned from a single reflecting object or target. Signal E1occurs at a time after pulse P1 equalto the time interval R. Similarlypulse E2 occurs after pulse P2 and pulse E3 occurs after pulse P3 by thesame time interval R. It is this time interval R that indicates therange from the radio system of the target producing pulses E1, E2, andE3. Fig. 3G illustrates the output of delay line I: or amplifier I6 ofFig. 1'. Pulse P1 occurs at a time after pulse P1 in Fig. 3F equal tointerval D2 and pulse P2 occurs at the same time interval D2 after pulsePzin Fig. 3F. Pulses E1 and E2 occur after P1 and P2 respectively, byatime interval equal to time interval R of Fig. 3F. The time interval,therefore, between pulse E1 and pulse E1 and pulse E2 and pulse E2 willalso be equal to the time interval D2. be greater than the maximumspacing in time between successive pulses in Fig. 3A. I

Fig. 3H illustrates the'signals in the output of delay line 54 andamplifier I8. The signal's'at these two points occur at identical times.Pulse P2. occurs after pulse P2 of Fig. 3F by a time interval D3 whichis the time delay of delay line It. In a similar manner pulse E2" ofFig. 3H occurs at. a timeinterval D3 after pulse E2 of Fig. 3F whereagain interval. D3. is equal to The time delay D2 of delay line I2should 6" the time delayof. delay line. MI The circuit has been soadjusted that pulses P2" and P1"..correspond in time and also pulses E2"and E1'correspond in. time. This may. be. accomplished by properlyselecting thepotential to which charging circuit 38 will 'chargewhenactivated by a voltage gate having a width C1.

this exampleDa' is greater than D3 due to the fact that the potential onthe cathode 42 of de.-. lay line' I4 is lowerafter pulse T3 than it isduring. the time between pulse T2.and pulse T3. Again,'delajy D3 is justsufficient to cause pulse P3 and pulse E3 to correspond in time to P2and E2 respectively of Fig. 3G. .If. the circuit constants ofchargingcircuit 38 and delay line I4 are properly selected the intervalD3, D3 and other corresponding delay inter: vals produced by delay lineI4 will be just that value necessary to cause, coincidence. betweencorresponding pulses in Figs. 3G and 3H, that is to say,.correspondingpulses at the two inputs to comparison circuit 20 of Fig. 1 will occurat identical times. If pulses E1, E2 and E3 represent a moving target anoutputwill be obtained from comparison circuit 20 at output 22. If,however, these pulses represent a stationary target, no output will beobtained at connection 22. As .far asthis invention is concerned,however, it makes no difference whether the pulses represent a movingtargetor a stationary target since this invention deals only with thespacing between pulses and not the amplitude of the pulses.

Some of the advantagesof this-circuit are: r

(1) A spark gap modulator may now be employed in a radio objectlocatingsystem if this invention is also employed; 2

(2) The circuit of thisinvention may be employed in any type ofmovingtarget radio object locating system to compensate for the variations inspacing in time of successive pulses transmitted by the system;

Other advantagesof this circuit will beimmediately obvious to thoseskilled'in the art.

While there has been described what is at presfications may be madetherein without departing I from the scope of the invention as set forthin the appended claims. x a What is claimed is:

1. In a radio object locating system, the combination including asparkgap modulator constituting a first source of pulse signals, a radiopulse receiver constituting a second source of pulse signals, and acomparison circuit having two inputs thereto, a pulse coincidencecircuit comprising a first and a second fixed delay line, a variabledelay line, means for applying pulse signals from said second source tosaid variable delay line and to said first fixed delay line, means forapplying the output of said variable delay line to one input of saidcomparison circuit, means for applying the output of said first fixeddelay to the remaining input of said comparison cir- Pulse Psfi'anpulseEafioccurafter pulsesPa and pulse E3 re spectivelyof .Fig. 3F byatime delayv D3 where D3 is thenew time delayof delay line I4... In

pulses v aerate-r.

7 maining input of said control voltage. circuit through .saidsecondfixed delay line, means for applying the output of said control voltagecircuit' to .s'aid variable delay line in:a manner tocontrol thedela'y'time thereof so that selected signals present at the firstinputof said comparison :circuit correspond :in time, to :other selectedsignalsipresentvat the, second-input to said comparison circuitregardless of the spacing of pulses in the output of said modulator.

2. A devicefor obtaining coincidence of sigr als .in the receiverchannel of a moving target radio locating system"utilizing variably:spaced exploratory pulses, said'device including means for delayinggiven signals in the output of said.

' system so that said given signals correspondlin time to saidsucceeding signals.

3. A device for obtaining coincidence ofselected signals of first andsecond groups of signals derived from third signals, said deviceincluding means for delaying the first :group of signalsby a fixedamount, and .means for delaying the second group of signals by .avariable amount controlled by the "time spacing of the third signals sothat said selected signals in said first and said second group ofdelayed signals occur in time coincidence.

4. In apparatusincluding a :first source :of

pulse signals anda second source 'oftpulsesignals wherein the time ofoccurrence of signalsrfrom said second source is determinedinpart bysignals from said pulse source, a circuit for causing selected signalsin the output of said. second pulse source to occur in time coincidencewith other selected signals in the .output of said second source, saidcircuit comprising :a first, a second, and a third delay means, meansfor applying the output of said second pulsesourceto the inputs of saidfirst and said second delay means, means for generatinga'voltagerepresenting the interval between a pair of pulses, means forapplying the output of said first pulse source directly to one input ofsaid voltage generating means, means for applying the output of saidfirst pulse source to a second input of said voltage generating 1 meansthrough said third delay line, and means for applying the outputofisaid'voltage generating means to said second delay linefto'controlthe delay thereof so that the above-mentioned selected signals appearatthe outputs of said first and said second delay lines respectively inthe desired time coincidence.

5. A radar system comprising means/for initiating exploratory pulses, apulse echo receiver, circuit means for producing an output correspondingto the'voltage difference between a pair' of input signals impressedthereon, means connected to said receiver for impressing on said circuitmeans a received echo signal derived from a given exploratory pulse,variable delay means connected to said receiver for variably delaying areceived echo signal derived'fromian exploratory pulse adjacent to saidgiven exploratory pulse and for impressing said delayed signal on saidcircuit means, and means for varying the delay period of said'delaymeans'in response to the length of the interval between the said givenand adjacentexploratory pulses'so that one of said received-echo signalsis delayed with respect to'the 'other'received Zsignal forra time equalto the'interval between. said :exploratory pulses; y

6. In a moving object radio locating system, apparatus "fordistinguishing "fixed object from moving objectipulseechoes including apulse generator; alradioipuls'e receiver, and a pulse comparison:circuitzhavfing two inputs, thereto, a pulse coincidence' circuitcomprising an-fixed delay line,

a variable delay line, means for applying pulse signals :from "said:receiver .to :said variable delay line, :means for applying the outputof said variable; delay line to one input of said comparison circuit,means for applying, the output of said receiver :to the remaining inputof said comparison-circuit, a'-contro1 voltage circuit having two inputsfor providing a unidirectional potential corresponding amplitude to thespacing in time-of twoinput'pulse signala means for connecting said*pulse :generator to one input to said control voltage circuit, meansfor connectingxsaid pulsegenerator :to the remaining input of saidcontrol 'voltage'circuit through said fixed delay line, means forapplying the output of said controlvoltage-:circuit to said *va'r-iabledelay line to control the delay time thereof so that selectedsignals-present at the first input of said comparison circuitcorresponds in. time to other selected signals present at the secondinput :to said comparison circuit regardless of the spacing of pulses.in theoutput of said pulse generator. V

'Z. .In a radio object locating system, apparatus for distinguishingfixed object from moving objectpulse echoes including a pulse generator,a

radio pulse receiver, and 'a pulse comparison circuit having twoinputs'thereto, a pulse coincidence circuit comprising a variable delayline, means for applying pulse signals from said receiver to saidvariable delay line, means for applying the output of said variabledelay line to one input of said -:comparison circuit, means forapplying'the output of said receiver to the remaining input ofsaidlcomparsion circuit, a controlvoltagepircuitfor providing aunidirectional potential corresponding in:-amplitude to the spacing ;intime :Of two input pulse signals, means for connecting saidpulse-generator to an 'input to said control voltage circuit, means forapplying, the :output "of said control voltage circuit to saidyar-iabledelay line to control the delay time thereof so that selected pulse echosignals presentat the first input of said comparison circuit correspondsin time .to other selected pulse echo'signals present at the secondinput to said comparison circuit regardless "of the spacing of pulses inthe output of said pulse generator.

*8; The apparatus defined in claim 1 wherein said variable delay :lineincludes a positive ion delay'tube. T r 7 -9. In 'a-radio objectlocating system, apparatus for distinguishing between fixed object. andmoving object echoes including a pulse modulater; a radio :pulsereceiver, and a pulse comparison circuithaving two inputs thereto, ,apulse coincidence circuit-comprising a fixed delay line, a variabledelay line, means :for applying pulse signals from said receiver tosaidvariable delay line and'to said fixed delay line, means for applying theoutput of said variable delay line toone input of said comparisoncircuit, means for applying the output of said fixed delay line to theremaining input of said comparison cirwit, acont-rolvoltage circuit forproviding a unidirectional potential corresponding in amplitude'to thespacingiin time of two input pulse signals, means for connecting saidmodulator to said control voltage circuit, means for applying the outputof said control voltage circuit to said variable delay line to controlthe delay time thereof so that selected pulse echo signals present atthe first input of said comparison circuit corresponds in time to otherselected pulse echo signals present at the second input to saidcomparison circuit regardless of the spacing of pulses in the output ofsaid modulator.

10. In a radio object locating system, the combination including a pulsemodulator, a radio pulse receiver, and a pulse comparison circuit havingtwo inputs thereto, a pulse coincidence circuit comprising a fixed delayline, a variable delay line, means for applying pulse signals from saidreceiver tosaid variable delay line and to said fixed delay line, meansfor applying the output of said variable delay line to one input of saidcomparison circuit, means for applying the output of said fixed delayline to the remaining input of said comparison circuit, means connectingsaid modulator to said variable delay line for controlling the delaytime thereof so that selected pulse echo signals present at the firstinput of said comparison circuit corresponds in time to other selectedpulse echo signals present at the second input to said comparisoncircuit regardless of the spacing of pulses in the output of saidmodulator.

11. A radar system comprising means for initiating exploratory pulses, apulse echo receiver, first means for delaying a received echo pulsederived from a given exploratory pulse for a period greater than theinterval between said given exploratory pulse and the next succeedingexploratory pulses, second means for variably delaying a received echopulse derived from said succeeding exploratory pulse, means for varyingthe delay period of said second delay means in response to the length ofsaid interval, said first and second means having inputs connected tosaid receiver, and circuit means connected to said first and secondmeans for producing an output corresponding to the difference betweenthe outputs of said first and second means.

12. The system defined in claim 11, wherein said second delay meansincludes a positive ion discharge tube having positive ion velocitycontrol means.

13. The system defined in claim 12 including means connected to saidpositive ion tube for controlling the positive ion current therein inresponse to received echo pulses.

ALFRED G. EMSLIE.

References Cited in the file of this patent UNITED STATES PATENTS ForbesDec. 5, 1950

